The annual change in daylength is the major environmental factor regulating the timing of reproduction in many seasonally breeding animals. The photoperiod regulates pituitary gonadotropin release in mammals through a series of neuroendocrine events involving the eyes, a neural circadian clock, the pineal gland and the release of hypothalamic GnRH. In the present proposal the role of these various components in the photoperiodic control of reproduction will be examined. The rapid and dramatic increase in serum FSH following exposure of Djungarian hamsters to a few cycles of brief light pulses during the night makes this an ideal species to examine photoreceptor-pineal-pituitary relationships. The effect of light pulses of different wavelength and energy on pituitary FSH release will be determined in order to characterize the properties of the photoreceptors responsible for mediating the effects of light on the reproductive system, and to compare these with the properties of the photoreceptors mediating the effects of light on pineal melatonin levels and the circadian rhythm of locomotor activity. The larger size of the golden hamster, as well as preliminary data on hypothalamic GnRH release and the pulsatile pattern of pituitary gonadotropin secretion in this species, makes it a more suitable model to examine the role of hypothalamic GnRH In the photoperiodic response. Hypothalamic GnRH release (measured via the push-pull perfusion technique) and serum gonadotropin levels (measured via an indwelling intra-atrial cannula) will be monitored simultaneously in the same animal repeatedly for 7-24 hours to determine if photic-induced changes in pituitary LH and FSH release are associated with changes in hypothalamic GnRH release. These studies will be carried out in intact, castrated and castrated- testosterone treated animals to determine the steroid-dependent and steroid-independent effects of the photoperiod on hypothalamic-pituitary activity. Hypothalamic GnRH release will also be monitored in pinealectromized, melatonin-treated or SCN-lesioned animals to determine if these treatments alter GnRH secretion in the same manner as light. The results obtained from these studies are expected to provide new insight into the neural and endocrine mechanisms underlying the effects of light on the circadian and reproductive systems and should aid in the understanding of various disorders that are related to dysfunction of these systems.